Myxococcus xanthus cells exhibit coordinated cell movements on a solid surface during vegetative growth and multicellular fruiting body formation. These complex social behaviors make this bacterium an excellent model system for studying intercellular signaling and microbial development. The focus of our research is to understand the mechanical and physiological basis of social gliding motility (S-motility) and the chemotactic control of motility during fruiting body formation. During the last grant period, we discovered some important molecular functions of two cell surface appendages that are required for social motility: the type IV pili and fibrils. Based on these findings, we hypothesize that directed motility in M.xanthus involves the control of pilus switching frequency by the frz chemosensory system. We also hypothesize that fibril, a self-generated extracellular matrix material, is a major chemoattractant for M. xanthus and provides an important signal for fruiting body formation in M. xanthus. We propose the following specific aims to test these hypotheses: 1. to obtain direct physical evidence of pilus dependent motility and study its interaction with the frz chemosensory system; 2. to obtain direct visual evidence of fibril-guided chemotactic movement during fruiting body formation; 3. to identify molecular components involved in self-generation of fibril gradient. The studies will help us to understand the molecular mechanisms of social swarming, social hunting, intercellular signaling, and fruiting body formation. It will also provide a molecular understanding of gliding motility and the evolution of bacterial motility/chemotaxis systems. Since the S-motility and aggregation of M. xanthus are very similar to twitching motility and biofilm formation in pathogenic bacteria like Pseudomonas and Neisseria, the studies may also provide clues for further molecular characterization of these events, leading to new treatments against these pathogenic bacteria.